1. Technical Field of the Invention
The present invention relates to networked communications systems, and in particular, to apparatus and methods operating within a communications network providing the ability to transport long messages via segmentation capability testing.
2. Description of Related Art
Within a communications network messages are passed between terminal points, which may also be characterized as nodal entities. The usual order of communication involves a message sent from a first node to a second node, and a response or reply to the initial message, sent back from the second node to the first node. For various reasons, such as the presence of excessive electronic noise or a physical fault in the network, the response may never reach the first node.
Timers have been built into network nodes to ensure that the failure to receive a response does not result in network inactivity for an indefinite period. That is, when a message is sent from one node to another, a timer within the sending node is typically started and allowed to run for a preselected period of time. If no response is received before the timeout period (i.e., when the timer counts down to a value of zero), then the sending node is alerted to this fact and appropriate recovery measures can be taken.
One reason for the failure of nodes in a network to receive a response is the existence of increasingly sophisticated application software, which requires sending messages exceeding the limits established by current standards. For example, the Common Channel Signaling System No. 7 (SS7) standard is a global standard for telecommunications defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and the American National Standards Institute, Committee T-1 (ANSI T-1). The SS7 standard defines procedures and protocol wherein network elements in the Public Switched Telephone Network (PSTN) exchange information over a digital signaling network to effect wireless (i.e., cellular) and wireline call set up, routing, and control. SS7 messages are exchanged between network elements, or nodes, over bidirectional channels called signaling links. The signaling points, or nodes, in an SS7 network comprise Service Switching Points (SSP), Service Transfer Points (STP), and Service Control Points (SCP).
The hardware and software functions of the SS7 protocol are divided into functional abstractions called “levels.” These levels map loosely to the Open Systems Interconnect (OSI) seven-layer model defined by the International Standards Organization. The lowest level, or Message Transfer Part (MTP) is divided into three sub-levels. The lowest level, MTP Level 1, is equivalent to the OSI Physical Layer. MTP Level 1 defines the physical, electrical, and functional characteristics of a digital signaling link. The next level, MTP Level 2, insures accurate end-to-end transmission of a message across the signaling link. Thus, MTP Level 2 implements flow control, message sequence validation, and error checking. MTP Level 2 is equivalent to the OSI Data Link Layer. Finally, MTP Level 3 provides message routing between signaling points in the SS7 network. MTP Level 3 re-routes traffic away from failed links and signaling points, and controls traffic when congestion occurs. MTP Level 3 is equivalent to the OSI Network Layer.
Within MTP Level 2, an SS7 message is called a Signal Unit (SU). There are three kinds of signal units: Fill-In Signal Units (FISUs), Link Status Signal Units (LSSUs), and Message Signal Units (MSUs). MSUs carry all network management, control, database query and response, and network maintenance data in the Signaling information Field (SIF). MSUs have a routing label which allows an originating signaling point to send information to a destination signaling point across the network. The SIF in an MSU contains the routing label and signaling information.
The Signaling Connection Control Part (SCCP) Level provides connectionless and connection-oriented network services and global title translation capabilities above MTP Level 3. The SCCP Level is loosely equivalent to the OSI transport layer. While MTP Level 3 provides point codes to allow messages to be addressed to specific signaling points, the SCCP provides subsystem numbers to allow messages to be addressed to specific applications (called subsystems) at the signaling points. The SCCP also provides the means by which an STP can perform global title translation, wherein the destination signaling point and subsystem number are determined from digits present in the signaling message. Because an STP provides global title translation, originating signaling points do not need to know the destination point code or subsystem number of the associated service. Only the STPs need to maintain a database of destination point codes and subsystem numbers associated with specific services and possible destinations.
SCCP messages are contained within the Signaling Information Field (SIF) of an MSU. The SIF contains the routing label followed by the SCCP message content. The SCCP message is comprised of a one-octet message type field followed by the mandatory fixed-part, mandatory variable part, and an optional part. Each optional part parameter is identified by a one-octet parameter code followed by a length indicator (i.e., “octets to follow”) field.
The Transaction Capabilities Applications Part (TCAP) Layer sits on top of the SCCP Level and supports the exchange of non-circuit related data between applications across the SS7 network. The SCCP is used as the transport layer for TCAP-based services. For example, in mobile networks, TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming.
The TCAP Layer enables the deployment of advanced intelligent network services by supporting non-circuit related information exchange between signaling points using the SCCP connectionless service. For example, an SSP uses the TCAP Layer to query an SCP to determine the routing numbers associated with a dialed “800” number. The SCP also uses the TCAP layer to return a response containing the routing numbers (or an error or reject component) back to the SSP. Similarly, when a mobile subscriber roams into a new Mobile Switching Center area, the integrated Visitor Location Register requests service profile information from the subscriber's Home Location Register using MAP information carried within TCAP messages.
TCAP messages are contained within the SCCP portion of an MSU. A TCAP message comprises a transaction portion and a component portion. The transaction portion contains the package type identifier (e.g., query with permission, conversation with permission, etc.). The component portion contains components, such as Invoke (Last), Return Result (Last), Return Error, and Reject. Components include parameters containing application-specific data carried unexamined by TCAP Layer.
Since the Length Indicator (LI) portion of the MSU is limited to values between 0 and 63, a LI of 63 indicates that the message length is equal to or greater than 63 octets (up to a maximum of 272 octets). The maximum length of a signal unit is therefore 279 octets: 272 octets (data)+1 octet (flag)+1 octet (Backward Sequence Number+Backward Indicator Bit)+1 octet (Forward Sequence Number+Forward Indicator Bit)+1 octet (LI+2 bits spare)+1 octet (Service Information Octet)+2 octets (Cyclic Redundancy Check). The problem addressed by the present invention arises when message data length exceeds the 272 octets allowed by a MSU.
Currently, for TCAP messages having lengths of greater than 272 characters (derived from an application operating within a signaling node), the SCCP Layer will act to divide the message data length into portions which are 272 octets long, or less. When the entire message data can be carried in a single MSU, the message is known as a Unitdata Message (UDT). When the message length is such that multiple MSUs are required to transport the message, the individual MSUs are known as Extended Unitdata Messages (XUDTs).
Unfortunately, the SCCP Layer in older nodes does not provide a useful return result when presented with an XUDT message. For example, when a QUALREC message having more than 272 characters of message data is sent to an older SCCP node, no qualrec-—return_result (or return_result or reject) message is returned to the sending node. Instead, such nodes discard XUDT messages. The application layer in the network is not aware of exactly why no return result message was obtained; the application is typically only able to indicate that there is a problem with the network communications. Thus, the failure to return a result may be characterized as a network noise problem, a hardware failure in the receiving node, or any number of other problems. The true cause of the error, which is the inability of the receiving node to properly process XUDT segmented messages provided by the SCCP, is not communicated to the sending node.
Therefore, what is needed, is a system and method supporting message transport and segmentation in a communications network having a plurality of nodes, wherein some of the nodes are capable of processing segmented messages, and other nodes are incapable of processing such messages. The system and method should be inexpensive to implement and relatively robust in operation. Further, the system and method should support testing and recordation of message segmentation and transport capability within the network.
Finally, a node supporting message transport and segmentation within a communications network is also needed. The node should have the capability of testing and recording the ability of other nodes to transport and segment long messages. The system, method, and node of the present invention should all support message transport and segmentation using the OSI model, and more particularly, the SS7 protocol standard.